remove ltdl

ltdl is a pain on windows and unneeded on all other platforms. Why keep it if it is unneeded/easily replaced? On OS X 10.8 apple stopped providing ltdl but dlopen/dlfcn.h is available and the recommended way: https://developer.apple.com/library/mac/#documentation/developertools/Reference/MachOReference/Reference/reference.html

I have also researched other toolkits and how they support dynamic loading in a cross platform way and tools like gdb, llvm, and node.js/libuv also take this approach of using windows apis directly and otherwise just dlopen.

One thing this patch does not do is leverage/report any errors from `dlopen`. Honestly I don't think this is critical. Ltdl provided an api to get errors but it was so buggy I ultimately disabled it. So, not accessing errors from dlopen/LoadLibrary does not regress any functionality at all. In the future we could consider adding this if it seems needed, but I think we should not mess with custom errors for now.